How we filmed The Invisible Man – as told by the cast and creative team

"A lot of times actors are talking to somebody, and they're having a whole speech to some monster... but on the set they're talking to a tennis ball on a C-stand," wrier/director Whannell tells RadioTimes.com. "You know, that's a common thing."

In this movie, though, the threat facing Moss’s Cecilia is one that will never be added later by a hardworking VFX artist. Her abusive ex-boyfriend Adrian (Oliver Jackson-Cohen) is tormenting her while invisible, leading to a film full of Moss nervously looking around apparently empty rooms, with only the odd footstep, creak or moving bit of furniture offering a clue as to where he could be.

"Sometimes in movies you're acting to somebody you'd rather not be acting at all with, so this was in a way a different kind of challenge than that," Moss laughed when we asked about the filming process.

But how did the team pull it off? And was there ever actually someone there? Well, as in the film itself you can't always trust what you see.

Whannell and his crew used a combination of old-school techniques and up-to-date CGI wizardry to bring the Invisible Man to life, with some scenes requiring a fully-green-suited figure that could be painted out later and others achieved with nothing more than a simple bit of string.

"Well with, say, the fight scenes, that was a real mixture of things," Whannell said.

"We had Lizzie being pulled around in wires. We had a stunt person in a green suit, who then had to be removed digitally.

"But then also in those scenes we would also use really old school practical effects like pulling doors closed with a piece of string.

"Some props guy would be hidden in a cabinet, and he would pull this piece of string and a door would close or a cabinet would open. It made you realise that how you do a visual effect doesn't matter - it's only the end result that matters."

"You know I learned that on Paranormal Activity, the first Paranormal Activity," producer Jason Blum agreed, noting with some irony that the Invisible Man was the "perfect" monster for the smaller-budgeted films created by Blumhouse Productions.

"The effects of Paranormal Activity were all done by [director] Oren and his neighbour. And it's as scary a movie as there's ever been! I really internalised those lessons. Simple is generally scarier."

Often, then, Moss genuinely was just emoting to an empty room with no-one to bounce off – though for certain scenes, she insisted that co-star Jackson-Cohen be present to help lend authenticity to the performance.

"Leigh and I were trying to be specific about when it could be the stunt double, when it could be Olly, when it should be nothing," Moss explained.

"And we tried to really make sure there was specificity there. And there were times when if the Invisible Man had to speak in the scene, I would prefer to have Olly there.

"And then there were times when there was nobody there at all and it was just a blank space."

"I’m there more than you would think," Jackson-Cohen, who only physically appears briefly in the movie, told us. "We're trying to be quite selective about where we say parts of it are me or not, because we kind of want to keep an audience guessing as to how we did it.

"But it was definitely not... I didn't shoot for like four days on the movie. I was there for two months. So it was a fair amount.

"Lizzie and I spent some time discussing what parts of the script she really needed me to be there, and we felt would help performance-wise. So, yeah. Slip me into a green suit and I'm a go."

However, Moss admitted, it is possible that that Jackson-Cohen didn’t need to be on set quite as often as he was…

"The non-serious answer is then I just started asking Olly to come to set to entertain me, and to amuse me," she laughed.

"I just thought it was funny to look at. There's nothing like a tall man in a tight green-screen suit."

Once they had their unseen man, a bigger challenge came in how to communicate that he was actually there. For Whannell, this ended up being one of the key struggles in making the film work, and he attempted to achieve the effect through a combination of slow, roving camera angles (implied to be from the Invisible Man’s perspective), ingenious sound design and Moss’s performance alongside the carefully-implemented special effects.

"I wanted this character to be a real presence in the movie without being seen," he told us.

"So the question becomes, how do you make someone's presence in a movie feel very heavy and real without showing them?

"Do you use music? I guess in the case of Jaws, that music tells us the shark is there. Even though we don't see the shark, we hear the music. But I didn't necessarily wanna do that. I didn't want to have a theme song that would come on.

"So I don't know if I really succeeded, but at every step of the way I was trying to suggest the presence of somebody, without explicitly saying there's someone here."

For our money, he succeeded - and in the end, there’s something beautifully ironic about the fact that the biggest and most complicated bit of CGI work in this film ended up being how to hide the monster from the audience, rather than add a terrifying creature in to the action.

"Painting out the green suit is quite a difficult process," Whannell admitted.

"Yeah that was the hardest thing to do – I actually proved to be harder than we thought! Removing the quote/unquote Invisible Man was actually quite complicated," Blum agreed.

"But look, the scariest movies don't have a lot of special effects. And of all the monsters, the one that needs the least effects is clearly The Invisible Man.

"In movies you can throw a lot of money at crazy effects, make a crazy-looking monster or whatever. I think keep writing and keep pushing the character and the story and the performance, and your choice of actors.

"Those things are much more important for a good movie than big production value, I think."

The Invisible Man is in cinemas now